The vertical positioning roller shades is often controlled with a spring clutch mechanism. Such mechanisms are usually operated with a loop of cord or bead chain. A significant advantage of using loops to position shades is that it is not necessary to handle the shade material itself when operating the shade. This keeps the shade material much cleaner and, in the case of unsupported materials, prevents damage to the material.
However, with loop operated mechanisms it is possible to roll the material backwards onto the roller. This is undesirable in most cases, and in some cases it prevents the shade from working properly and can cause damage to the material. This is particularly true of multi-layered materials, many of which are damaged by reverse rolling.
Some rather awkward and complex systems have been devised to prevent shades from being rolled backwards. One prior art mechanism uses a roller having an intricate shape together with other parts to effect a stop which prevents the rotation of the roller past bottom in the reverse direction. A simple clutch mechanism has been needed to provided loop operation while preventing reverse rolling of the material.